More energy from the sun strikes the Earth in one hour than all the energy consumed on the planet in one year, yet solar electricity accounts for less than 0.02% of all electricity produced worldwide. The enormous gap between the potential of solar energy and its use is due, in part, to the cost/conversion capacity. The development of third generation solar cells (high efficiency plus low cost) is of paramount importance to both humanity and nature.
Solution-processability has been recognized as a feasible solution to cost issues, and novel mechanisms such as carrier multiplication a possible route to achieve higher efficiency levels. In both these aspects, there is potential in colloidal infrared quantum dots, such as lead selenide (PbSe) and lead sulfide (PbS). The addition of quantum dots to present cost-effective organic solar materials (i.e., polymers) could double power conversion efficiency to twelve percent due to the infrared absorbers' enhanced spectrum match with sunlight. Initial observation of carrier multiplication and recent confirmation of it in these quantum dots holds fundamental importance in current solar cell development.
However, present PbSe and PbS quantum dots research has seemed to hit a ‘bottleneck’, hindering the achievement of their full potential. Efficient photo-induced charge transfer has not been observed in these quantum dot composites, due largely to the lack of a measurement technique which would allow a clear separation between exciton dissociation and charge transport phenomena. This makes it challenging to gain detailed insight into either phenomenon, impeding rational design of absorber layers. Furthermore, the majority of the transport studies so far have been limited to the planar structure field effect transistors (FET), whereas an applicable quantum dot photovoltaic (PV) device is of sandwich structure, and it is known that the transport characteristics could be very different in these two structures.
Quantum dots are essentially nanocrystals consisting of tens to hundreds of atoms. FIGS. 1(A) and (B) illustrate the rock salt crystal structure of PbSe quantum dots, with quantum dot core 1 and ligand 2. Due to the nanocrystal's small size (smaller than the exciton Bohr radius of the bulk semiconductor), strong quantum confinement results in discrete energy levels and bigger band gaps compared with the respective bulk semiconductor. FIG. 2 shows the quantized energy levels of a PbSe quantum dot. The dashed line represents the gap state energy level found on IV-VI quantum dots. Infrared quantum dots such as PbSe or PbS have size-tunable band gaps ranging from 0.4˜1.1 eV. Consequently, their optical absorption covers solar spectrums from infrared to ultraviolet. The graph of FIG. 3 illustrates the absorption spectra of variously sized PbSe quantum dots. Arrows indicate the first excitonic peak (1Sh-1Se) in the infrared region. Another tunable factor in quantum dots comes from their passivation layer (ligands), which largely influences optical and electronic properties.
In general, the photovoltaic (PV) process in the quantum dots system (quantum dots with one or two other constituents) consists of four successive processes:    (i) absorption of photons, which creates excitons (bounded electron-hole pairs);    (ii) exciton dissociation (or photo-induced charge transfer) following the exciton diffusion to a region (for instance, the interface of two different components);    (iii) free carriers transport separately toward the anode (holes) and cathode (electrons), where    (iv) charge collection occurs.
As an example, FIG. 4 is a diagram of a hybrid PV device made of PbSe quantum dots and conducting polymer P3HT. FIGS. 5(A) and (B) show how photo current is generated in the hybrid PV device. FIG. 5(A) shows electron e transferring from the P3HT polymer to the PbSe quantum dot. FIG. 5(B) shows the hole h transfer from the PbSe quantum dot to the P3HT polymer. The hybrid device of FIG. 4 is more thoroughly described in Lewis, et al. (U.S. Pat. No. 8,183,082), which is herein incorporated by reference.
Despite being one of the most promising solutions for solar energy utilization, the present performance of such infrared quantum dot-based PV devices are far from their expectations. The two major causes for their relatively low efficiency have been recognized in current research:
(1) inefficient exciton separation at the quantum dot/constituent interface; and
(2) poor charge percolation pathways to the extracting electrodes.
During the colloidal synthesis process, certain ligands (usually TOPO or oleic acid) are used to passivate quantum dot surfaces to prevent aggregations. Incomplete passivation could result in surface trap sites, and with the bulky ligands serving as barriers for exciton dissociation at the quantum dot/constituent interface, photo-induced charge transfer (PCT) is hindered. The addition of quantum dots without the optimization of their interfaces with other constituent(s)—i.e., without the formation of separate percolation pathways for electrons-e and holes-h—causes huge loss of the photo-generated free carriers due to e-h recombination.
Various kinds of post-synthesis chemical treatments of quantum dots have proven to be efficient to enhance quantum dot transport properties without sacrificing their confinement uniqueness. Recently, a series of studies on quantum dot device physics regarding ligand exchange with butylamine have demonstrated improved infrared response of PbS quantum dot photovoltaic devices and photoconductors. Thermal treatment is another method to improve carrier mobility and conductance of PbSe NC film via enhanced interdots electronic coupling.